The art of diagnostic imaging exploits contrasting agents that in binding or localizing site selectively within the body, help to resolve the image of diagnostic interest. .sup.67 Gallium salts, for example, have an affinity for tumours and infected tissue and, with the aid of scanning tomography, can reveal afflicted body regions to the physician. Other contrasting agents include the metal radionuclides such as .sup.99m technetium and .sup.186/188 rhenium, and these have been used to label targeting molecules, such as proteins, peptides and antibodies that localize at desired regions of the human body.
As targeting agents, proteins and other macromolecules can offer the tissue specificity required for diagnostic accuracy; yet labeling of these agents with metal radionuclides is made difficult by their physical structure. Particularly, protein and peptide targeting agents present numerous sites at which radionuclide binding can occur, resulting in a product that is labeled heterogeneously. Also, and despite their possibly large size, proteins rarely present the structural configuration most appropriate for high affinity radionuclide binding, i.e. a region incorporating four or more donor atoms that form five-membered rings. As a result, radionuclides are bound typically at the more abundant low-affinity sites, forming unstable complexes.
To deal with the problem of low affinity binding, Paik et al (Nucl Med Biol 1985, 12:3) proposed a method whereby labeling of antibodies is performed in the presence of excess DPTA (diaminetrimethylenepentaacetic acid), to mask the low affinity binding sites. While the problem of low affinity binding is alleviated by this method, actual binding of the radionuclide, in this case technetium, was consequently also very low. The direct labeling of proteins having a high proportion of cysteine residues also has been demonstrated (Dean et al; WO 92/13,572). This approach exploits thiol groups of cysteine residues as high-affinity sites for radionuclide binding, and is necessarily limited in application to those targeting agents having the required thiol structure.
A promising alternative to the direct labeling of targeting agents is an indirect approach, in which targeting agent and radionuclide are coupled using a chelating agent. Candidates for use as chelators are those compounds that bind tightly to the chosen metal radionuclide and also have a reactive functional group for conjugation with the targeting molecule. For use in labeling peptide and protein-based targeting agents, the chelator is ideally also peptide-based, so that the chelator-targeting molecule conjugate can be synthesized in toto using peptide synthesis techniques. For utility in diagnostic imaging, the chelator desirably has characteristics appropriate for its in vivo use, such as blood and renal clearance and extravascular diffusibility.